Tap water treatment is a process of obtaining drinking water or industrial water from natural water sources such as river water, lake and marsh water and underground water, which are suspended water. Sewage treatment is a process of obtaining regenerated water for miscellaneous use by treating domestic wastewater such as sewage water, or obtaining dischargeable clarified water. It is essential for these treatments to remove suspended matter by carrying out a solid-liquid separation operation (clarification operation). In the tap water treatment, suspended substances derived from natural water source (clay, colloid, bacteria, and the like.) are removed. In the sewage treatment, suspended matter (sludge, and the like.) in treated water that is biologically treated (secondary-treated) by suspended matter, activated sludge and the like in sewage water is removed. Conventionally, these clarification operations have been carried out mainly by a precipitation method, a sand filtration method, or a coagulation sedimentation plus sand filtration method, and recently, a membrane filtration method is spreading. Examples of the advantage of the membrane filtration method include the following matters.
(1) Clarification level of the obtained water quality is high and stable (safety of the obtained water is high).
(2) The installation space of a filtration apparatus can be small.
(3) Automatic operation is easy.
For example, in the tap water treatment, the membrane filtration method is used, as a substitute for the coagulation sedimentation plus sand filtration method, or as a means for further improving water quality of treated water subjected to coagulation sedimentation plus sand filtration, for example, by installing at a rear stage of the coagulation sedimentation plus sand filtration. Also regarding the sewage treatment, use of the membrane filtration method is studied for separation of sludge from sewage secondary treated water and the like.
In these clarification operations by membrane filtration, a hollow fiber-shaped ultrafiltration membrane or microfiltration membrane (pore diameter in a range of several nm to several hundred nm) is mainly used. As a filtration system using a hollow fiber filtration membrane, there are two systems: an inner pressure filtration system that filters from an inner surface side towards an outer surface side of a membrane; and an external pressure filtration system that filters from an outer surface side towards an inner surface side. Among them, the external pressure filtration system that can take large membrane surface area on the side in contact with the suspended raw water and thus can reduce the suspended matter load amount per unit membrane surface area is advantageous. Patent Literatures 1 to 3 disclose a hollow fiber and a method for producing the same.
Clarification by a membrane filtration method has many advantages that conventional precipitation method and sand filtration method do not have as described above, thus, spread to a tap water treatment and a sewage treatment is progressing as a substitute technology or complementary technology of a conventional method. However, a technology for performing stable membrane filtration operation has not been established over a long period of time, and it inhibits wide spread of the membrane filtration method (refer to Non Patent Literature 1). The cause of disturbing the stability of membrane filtration operation is mainly deterioration in water permeability. First cause of deterioration in water permeability is clogging (fouling) of membrane due to suspended substances and the like (refer to Non Patent Literature 1). Also, the membrane surface was rubbed by suspended substance to be abraded, and it sometimes causes deterioration in water permeability.
Incidentally, as a method for producing porous membrane, a thermally induced phase separation method is known. In this production method, a thermoplastic resin and an organic liquid are used. As the organic liquid, a solvent that does not dissolve the thermoplastic resin at room temperature, but dissolves it at high temperature, specifically, a potential solvent is used. The thermally induced phase separation method is a method for producing a porous body by kneading a thermoplastic resin and an organic liquid at high temperature to dissolve the thermoplastic resin in the organic liquid, then cooling the solution to room temperature to induce phase separation, and further removing the organic liquid. This method has the following advantages.
(a) It becomes possible to form a membrane even with a polymer such as polyethylene in which there is no appropriate solvent that can be dissolved at room temperature.
(b) A membrane is formed by being dissolved at high temperature then cooled and solidified, thus particularly when the thermoplastic resin is a crystalline resin, crystallization is promoted during membrane formation, and a high strength membrane is likely to obtain.
Based on the above advantages, this production method has been often used as a method for producing a porous membrane (for example, refer to Non Patent Literatures 2 to 5).